Refractory coated article



V. J. DE SANTIS ETAL REFRACT'ORY COATED ARTICLE Filed Jan. 24, 1949'STEP-#l STEP #2 STEP #4 REF/FACTORY O/Y/f HFPL /E INVENTORS f//A/cEA/T Ioss/)NTIS F A HUNTER P JKRZAK ATTORNY Patented `une 22, 1954 UNITEDSTATES P'EENT OFFICE Hunter,

Lake Elu, Eil.,

and Charles P.

Majlrrzak, South Orange, N. J., assignors to International StandardElectric Corporation, New York, N. Y., a corporation of DelawareApplication January 24, 1949, Serial No. 72,403

(Cl. 14S-31.5)

14 Claims. l

rl'his invention relates to improvements in the protection of metallicsurfaces against adverse iniiuences at high temperatures. In a moreparticular sense, the invention is concerned with the provision of acoating upon a metallic surface which will protect the underlying metalagainst the adverse eects, particularly oxidation, which result fromexposure of the coated article to oxidizing or otherwise deleteriousatmospheres at temperatures of 750 C. or above.

Recent technological developments in the iieldV of gas turbines,jet-propelled and rocket-propelled missiles have resulted in the needfor new materials capable of resisting the high temperatures encounteredin operation of such devices, particularly materials capable ofWithstanding such very high temperatures over an extended period oftime. The temperatures encountered under these conditions are so muchhigher than those encountered in equipment of the types heretofore knownthat a Whole new field of technology has developed in this connection.

It has long been recognized that many of the high-inelting-point metalsin their elemental or in alloyed condition retain their tensile strengthto the extent of at least 5000 pounds per square inch at operatingtemperatures of as high as 800 C. and in some instances 1000 C. Amongthese refractory metals are tantalum (M. P. ca. 2770 C.), columbium (M.P. ca. 1950 0.), molybdenum (M. P. ca. 26'70" C.) and tungsten (M. P.ca. 3370" C.). However, in order that these metals may retain theirstrength at these high temperatures, it has been necessary that theother operating conditions, especially the atmosphere, be subject tocontrol in order that oxidizing gases and other substances capable ofreactinor With the metals at these temperatures may be excluded. Forinstance, even at 500o C. in an oxidizing atmosphere tungsten andmolybdenum form volatile oxides so rapidly that little is left of asubstantial mass of metal after less than 100 hours under theseconditions, and tantalum and columbium, when thus -l oxidized, becomebrittle in 48 hours or less. At higher operating temperatures, say 1000C., these efects are produced after merely a few minutes, and, reducingor neutral atmospheres no longer afford protection to the metals becausethen the metals Will reduce water Vapor or carbon monoxide present withresultant oxidation of the metal, and, if a hydrocarbon gas or othercarbonaceous material be present, the metals will decompose them withprogressive carbonization and resultant embrittlement of the metal.Inasmuch as hydrocarbon vapors and Water vapor are commonly, indeednecessarily, present in the atmosphere around hightemperature operatingparts of gas engines, it is obvious that these metals are unsuited tosuch use, more especially at temperatures materially exceeding about'750 C`. for, under these conditions, the deleterious effects producedby the surrounding atmosphere reacting with the metal cause far-reachingchanges Within the body of the metal resulting in structural distortionand lack of strength, of ductility and of resilience.

It has long been recognized that carbon is one of the mostthermo-resistant materials generally available. It is cheap, easilyWorked, generally available in large quantities, and can withstandtemperatures far exceeding those which result in the structuralenfeeblement of refractory metal articles. The physical strength ofcarbon is not apparently aiected to any material degree by elevatedtemperatures and thus this material has found high favor among workersfabricating apparatus to be used under these conditions. Carbonpossesses several disadvantages, however, Which have limited the extentto which it can be utilized in such equipment: the extreme brittlenessof carbon makes it very diicult to fabricate into delicate structures;furthermore if such structures are fabricated, they are very fragile andeasily damaged in transit or during normal handling under conditions ofuse. Additionally, carbon burns if heated above a dull red heat, about759 C., in an oxidizing atmosphere or in an atmosphere containing aread-ily-reducible, oxygencontaining compound such as Water vapor. Inthis connection it will be recalled.v that nearly all combustion gases,as in gas turbines for instance, contain water vapor and thus atelevated temperatures carbon would burn in these gases. A furtherdisadvantage of carbon is that it is a porous material quite permeableto gases and this permeability increases with increase of temperature.For these reasons and other reasons connected with particularrequirements of individual equipment, carbon has not been foundsatisfactory for use in many types of high-.emperature equipment,particularly equipment which of necessity must be structurally delicateWhile retaining substantial ruggedness so that it is not damaged undernormal condition of transport and use.

It has also been suggested to coat refractory metals mentioned abovewith various mixtures of silica or other refractory metal oxides butthis did not result in articles capable of withstanding temperaturesabove 750 C. because of reaction of the coatings with the underlyingmetal resulting in oxidation thereof, although this reaction proceeds ata relatively low rate at lower temperatures.

One of the objects of the present invention is to provide a method forprotecting the surface of a refractory metal exposed to very-hightemperatures so that deleterious atmospheric iniuences, especially thepresence of oxidizing and hydrocarbon gases, will not affect theunderlying metal.

A further object of the invention is to provide a composite structurethat can be utilized in equipment functioning at high temperatures whichretains its physical ruggedness and strength, even when exposed totemperatures of 1000" C. or materially thereabove, and which possessesthe desirable properties of the refractory metals without thedisadvantages, above mentions, that characterized them.

Other objects of the invention will be apparent to those versed in theart to Which the invention relates upon consideration of the fcllowingdisclosure.

The accompanying single figure of drawing is a diagrammatic sectionalillustration of the process and products, intermediate and final,constituting the subject matter of this invention.

Regarded in certain of its broader aspects, the present inventioncomprises providing, upon a base of refractory metal, a layer ofrefractory carbides, formed in situ upon the metal surface, over whichis applied an outer coating of a material capable of successfullyresisting high temperatures and which normally aects the base metal, butis prevented from doing so by the barrier layer of carbides.

In a more specific sense, the present' invention comprises a base oftantalum, molybdenum, tungsten or the like, including alloys thereof oralloys in which one or more of the mentioned metals is present in asubstantially predominant proportion, with a layer of carbidescomprising a carbide of one or more of said metals and silicon carbide,titanium carbide or zirconium carbide or a mixture or alloy of the same,formed, in situ, on the base metal and bearing an outermost layer ofrefractory material, such as a noble or refractory metal, particularly ametal of the platinum group, carbon or mixed carbides including metalliccarbides.

in accordance with this invention the desired protective coating isproduced upon the refractory metal substantially as follows: a layer ofrefractory oxide is applied to the metal surface by dipping, spraying orelectrophoresis, or in any other manner which can be used to produce vathin, though uniform, adherent lm of coating on the metallic surface;this coating is then sintered and partially dissolved or otherwiseintegrated with the surface by heating to an appropriately hightemperature, e. g., l7002000 C. in vacuo or inert atmosphere; thereafterthe sintered oxide coating is covered with a coating of carbon, either,per se, in divided state or in the form of a carbonaceous material, sothat the base metal carries a coating of oxide with an outside coatingof carbon or carbonaceous substance. The article so coated is then firedat a temperature of at least l700 C., preferably under vacuum, to causereaction between the oxide coating and 4 the overlying carbon coating,it 'being understood that the carbon content of the outside coating isso related to the oxide content of the underlying coating that thereaction between the carbon and the oxide does not fully spend thecarbon but instead an excess of carbon remains after the reaction hastaken place. This reaction between the refractory metal oxide and thecarbon results in the formation of a layer of mixed cr alioyedrefractory carbides, together with the excess carbon above mentioned,upon the refractory metal surface, the carbides being those of the basemetal and of the metal moiety of the oxide layer. This fired coating,which is tenaciously adherent to the base metal, is then provided with afurther coating of a material resistant to adverse atmosphericconditions at temperatures of 750 C. or above, including temperatures ofthe order of l000 C, or substantially thereabove, particularly platinumor a metal of the platinum group, if the final product is to be used inan oxidizing or carbonaceous atmosphere, or carbon if it is to be usedin an atmosphere that is inert to carbon at these temperatures. Thesupericial appearance of the finished product resembles that of anordinary carbon or metal article, depending on the material selected forthe iinal layer, but upon more detailed examination it is found thatunderneath the exterior portion of the coating lies a hard layer ofrefractory metal carbides having a metallic crystalline appearance,which is extremely resistant to high temperatures, e. g., a mixture oralloy of tantalum carbide with zirconium carbide is capable of resistingtemperatures up to about 3900 C. without substantial impairment of itsphysical strength or other properties. it is especially noteworthy inthis connection that the carbide layer itself, as distinguished from acarbon layer on the refractory metal surface, does not migrate atelevated temperatures into the body of the metal, nor does it allowoverlying normally migratory layers to migrate.

Referring now to the single figure of drawing, it will be noted that itillustrates in fragmentary, largely-diagrammatic, sectional view, anarticle provided with a protective coating pursuant to the presentinvention. The refractory metal base plate i, preferably formed oftantalum, columbium or the like, is provided with a refractory metaloxide coating 2, preferably zirconium dioxide, which can be applied byspraying, dipping or electrophoretically from an organic liquid bath.After this coating has been applied, it is consolidated upon the basemetal by sintering in vacuo at a temperature of at least about 1700 C.so that it forms a thin film approximately .0005 inch thick continuouslycovering the base.

The sintered coated base is then provided with a second coatingconsisting of carbon which can be applied by any conventional methodsuited for application of coating compositions, e. g. spraying, dippingor electrophoretic deposition, so that there is provided upon therefractory oxide layer a carbon coating 3 approximately .003 inch thick.The coated article is then red at a temperature of at least 1700o C. ina vacuum whereby the carbon, which is present in excess of thestoichiometric proportion, reacts with the refractory Oxide to produce alayer of carbides, and a further layer 5 by reaction of carbon with theimmediately subjacent refractory metal base to produce a layer of mixedcarbides or alloy of carbides of the refractory metal base and the metalmoiety of the refractory oxide. The completion cf these reactions may bereadily detected by absence of oxides of carbon from the vacuum outlet.

Thereafter a further coating 6 of refractory metal or carbon is appliedand consolidated to yield a finished article which consists of therefractory metal base i bearing an immediately adjacent layer 5 composedof mixed or alloyed car bides with anv overlying layer 4 of refractorycarbides with an outermost layer 6` of refractory metalor carbon.Migration of the material from the layer B into the refractory metalbase l is prevented by the intervening layers 4 and 5 containingrefractory metal carbides.

To facilitate an understanding of the subject matter of the presentinvention certain specic embodiments thereof will be hereinafterdescribed in detail but it is clearly to be understood that theseexamples are provided by way of illustration, not by way of limitation,of the present invention.

Example I An article of tantalum is coated with silica by making thearticle an electrode in electrophoresis of a bath containing finelydivided particles of silica. When a coating of the desired thickness hasbeen obtained, i. e. a thickness of about .003 inch, the article isremoved from the bath, then dried and fired at a temperature of about1000o C. This causes the silica coating to become sintered'. Aftercooling, a coatingof carbon is applied over the silica coating and thenthe coated article is rered in Vacuum at about 1.500c C. until thereaction between the carbon and the silica is completed to form abarrier layer or silicon carbide on the metal base. It is4 to beunderstood that the amount of carbon coating applied to the silica issuch as to exceed that minimum amount necessary to cause full conversionof the silica to silicon carbide and under these conditions there isproduced upon the metal base acoating comprising a mixture of siliconcarbide and tantalum carbide. After a layer of a metal of the platinumgroup or car- :bon has been applied to this mixed carbide layer by aconventional method, for instance spraying, sputtering, electrophoresis,dipping, painting, etc., there is obtained a coated article which isresistant to temperatures up to about '750 C. At temperatures above thispoint there is some,

though almost insubstantial migration of the f carbide layer into theinterior of the underlying metal with resultant impairment of theresistance of the coated metal to migration of the outer coating at thehigh temperatures involved.

Example II An article formed of molybdenum is treated as described inExample I except that all rings are at slightly lower temperatures thanare there indicated, so that there is produced upon the article abarrier coating of the mixed carbides of molybdenum and silicon. Thiscarbide barrier coating possesses substantially the samecharacteristics, physically and chemically as are possessed by thetantalum carbide silicon carbide barrier coating obtained by practice ofthe process described in Example I.

Example III The process as described in Example. I is repeated exceptthat the silica is replaced with zirconia and the iirst and second ringsare at a temperature of 2000 C. This results in an article bearing abarrier coating of the mixed or alloyed carbides of tantalum andzirconium which is resistant to atmospheric conditions at temperaturesof well above '750 C. or thereabouts. Otherwise theproperties of thefinished coated article are substantially those as set forth in ExampleI.

Example IV The procedure of Example III is. repeated except. thatmolybdenum is substituted for the tantaluln as the base metal and exceptthat all iirings are at slightly lower temperatures than are thereindicated. Under these circumstances, the barrier coating is a mixtureof the carbides of zirconium and molybdenum which is resistant toelevated temperatures up to. well above about 750 C.

Example V The process as described in Example I is repeated except thatthe silica is replaced with titania, and the first and second firingsare at a temperature of 2000o C. This results in an article bearing abarrier coating of the mixed or alloyed carbides of tantalum andtitanium which is resistant to atmospheric conditions at temperatures ofwellv above '750 C. or thereabouts. Otherwise the properties of thefinished coated article are substantially those as set forth in ExampleI.

Erample VI The procedure of Example V is repeated except that molybdenumis substituted for the tantalum as the base metal and except that allfiring-s are at slightly lower temperatures than are there indicated.Under these circumstances, the barrier coating is a mixture of thecarbides of titanium and molybdenum which is resistant to elevatedtemperatures up to well above about 75@0 C.

It will be recognized by those skilled in the art to which thisinvention relates that the invention is not limited to any specic mannerof applying the successive coatings on to the refractory base. Thus thecoatings of 'the refractory oxide, of the carbon or carbonaceousmaterial and the final` coating of refractory metal of the platinumgroup or carbon may be effected by any procedure known in the art forproducing Such coatings including the operations of dipping the articlein a coating composition, forming the coating by applying a coatingcomposition by spraying or producing a coating by electrophoresis of abath containing charged particles of the coating substances in a sizesuited for electrophoretic deposition. The initial sintering of theoxide coating can be effected within a substantially wide range oftemperatures provided the temperature is sufficient to effect asatisfactory integration of the particles of refractory oxide with theunderlying core so that during subsequent, processing whereby the oxideis converted to the carbide, a coating of carbides is obtained upon theunderlying metal. For this purpose, temperatures of about 1500" C. forsilica, about 2000* C. for zirconia and titania on a tantalum base,v orlower temperatures on a molybdenum base, have been found to be quiteadequate but higher or lower temperatures may be used. if peculiarconditions of operation make these changes desirable. The final firingoperation whereby the stratied refractory metal oxide and carboncoatings are converted into a refractory metal carbide layer on therefractory metal surface may be effected at a temperature approximating,that used in the first firing' operation. It is desirable, though notessential, that the second firing take place either under vacuum or in anoble gas which facilitates conversion of the refractory metal oxide tothe corresponding carbide.

While from the foregoing it will be understood that the refractory metalcarbide layer produced upon the refractory metal base pursuant to thisinvention inherently possesses entirely satisfactory resistance tomigration by an overlying layer of a platinum group metal at very hightemperatures, nevertheless a layer of carbon may in some instances bepreferable to the layer of platinum group metal and in this instance toomigration is prevented. The article having an outermost layer of aplatinum group metal applied on top of the carbide layer possesses tothe fullest measure the qualities of resistance to unfavorableatmospheric conditions which are characteristic of the metal andpermanently retains these properties inasmuch as migration is prevented.One explanation of these phenomena is as follows: the mixed or alloyedcarbides in the barrier layer are chemically inert toward the core metaleven at the high operating temperatures involved at which the core metalnormally is very reactive toward carbon, thus the barrier remainssaturated with carbon and thus does not migrate. The result is that asurface coating which normally would migrate into the underlying metalis excluded effectively from such migration and so remains on thesurface to resist oxidation, etc., as if the entire body were formedfrom the same substance as this outside layer.

In our joint application, Serial No. 72,404, filed January 2li, 1949,now Patent 2,552,535, we disciose an electrode for electron dischargedevices, wherein a base metal core is nrst coated with a barrier layer,similarly as disclosed in the present application, over which a layer ofnon-emissive material is applied, the barrier layer preventing migrationof the non-emissive material to the base metal core oi the electrode.

It will be understood that in the foregoing specification and in thefollowing claims the term refractory metal includes the elementsmolybdenuni, tantaluin, tungsten and columbium tgether With mixturesthereof and alloys in which one or more of these metals is thepredominant constituent imparting its characteristics to the alloy. Theterm refractory oxide includes zir conium oxidey titanium dioxide andsilicon dioxide or mixtures thereof with similar or related refractoryoxides wherein the zirconium oxide or the silicon dioxide is the chiefor predominant component imparting its qualities as the principalproperties ci the mixture. Vhen reference is made herein to carbon eachof the elemental ailotropic forms of the element is meant, although forobvious practical considerations graphite or soot is the form in whichthe element is most likely to be used when producing the carbon coatingon the refractory oxide or mixed refractory metal carbide coating.

While We have described above the principles of our invention inconnection with specic apparatus, it is to be clearly understood thatthis description is made only by Way of example and not as a limitationto the scope of our invention.

We claim:

l. A composite structure that comprises a refractory metal base formedof a metal chosen from the class consisting of tantalum, columbium,molybdenum, tungsten and alloys in which at least one of said metals isthe predominant component and a hard crystalline barrier layer 8 uponthe base that comprises refractory carbides consisting essentially ofzirconium carbide and a carbide chosen from the class consisting of thecarbides of tantalum, columbium, molybdenum and tungsten.

2. A composite structure that comprises a refractory metal base formedof a metal chosen from the class consisting of tantalum, columbium,molybdenum, tungsten and alloys in which at least one of said metals isthe predominant component and a hard crystalline barrier layer upon thebase that comprises refractory carbides consisting essentially ofzirconium carbide and a carbide of at least one of said base metals.

3. A composite structure as dened in claim 1 in which the refractorymetal is tantalum and one or the carbides is a carbide of tantalum.

4. A composite structure as dened in claim 1 in which the refractorymetal is columbium and one of the carb-isles is a carbide of columbium.

5. composite structure as defined in claim 1 in which the refractorymetal is tungsten and one of the carbides is a carbide of tungsten.

6. A composite structure as defined in claim 1 in which the refractorymetal is molybdenum and one or" the carbides is a carbide of molybdeum.

7. 'lin a process for producing a protective coating' upon a refractorymetal base chosen from the class consisting of tantalum, columbium,molybdenum, tungsten and alloys in which at least one of said materialsis the predominant component, the steps of applying to said base acoating oi iinely divided zirconium oxide, firing to cause sintering ofthe oxide coating, applying a layer of carbon on to the sintered oxidecoating, and then firing at a temperature of at least C-D C. to producea coating of carbidcs of the base metal and of zirconium.

8. Process as defined in claim 7 in which the refractory metal of thebase is tantalum.

9. Process as defined in claim 7 in which the refractory metal of thebase is columbium.

10. Process as defined in claim 7 in which the refractory metal cf thebase is tungsten.

1l. Process as defined in claim 7 in which the refractory metal of thebase is molybdenum.

12. rThe method or providing a molybdenum body with a resistant coatingwhich comprises depositing on the surface of said body the elementzirconium under conditions producing a molybdenum-zirconium layer onsaid body and reacting the said layer with carbon to produce amolybdenum-zirconium carbide layer integrally bonded to the molybdenumbody.

13. A molybdenum body having a resistant coating therein consistingessentially of the carbides of molybdenum and zirconium integrallybonded to the molybdenum body.

14. A molybdenum body having a resistant coating thereon consistingessentially of the carbides of molybdenum and silicon integrally bondedto the molybdenum body.

References Cited in the rile of this patent UNITED STATES PATENTS NumberName Date 1,362,138 Elsey June 7, 1932 2,051, 2S Dester Aug. 25, 19362,497,110 Williams Feb. 14, 1950 FOREIGN PATENTS Number Country Date842,981 France June 22, 1939

1. A COMPOSITE STRUCTURE THAT COMPRISES A REFRACTORY METAL BASE FORMEDOF A METAL CHOSEN FROM THE CLASS CONSISTING OF TANTALUM, COLUMBIUM,MOLYBDENUM, TUNGSTEN AND ALLOYS IN WHICH AT LEAST ONE OF SAID METALS ISTHE PREDOMINANT COMPONENT AND A HARD CRYSTALLINE BARRIER LAYER UPON THEBASE THAT COMPRISES REFRACTORY CARBIDES CONSISTING ESSENTIALLY OFZIRCONIUM CARBIDE AND A CARBIDE CHOSEN FROM THE CLASS CONSISTING OF THECARBIDES OF TANTALUM, COLUMBIUM, MOLYBDENUM AND TUNGSTEN.